This section describes commands that run other commands in some context different than the current one: a modified environment, as a different user, etc.
env: Run a command in a modified environment
env runs a command with an environment modified as specified
by the command line arguments.
Synopses:
env [ option ]... [ name=value ]... @c [ command [ args ]... ] env
Arguments of the form `variable=value' set the environment variable variable to value value. value may be empty (`variable='). Setting a variable to an empty value is different from unsetting it.
The first remaining argument specifies the program name to invoke; it is
searched for according to the PATH environment variable. Any
remaining arguments are passed as arguments to that program.
If no command name is specified following the environment
specifications, the resulting environment is printed. This is like
specifying a command name of printenv.
The program accepts the following options. Also see section Common options.
nice: Run a command with modified scheduling priority
If no arguments are given, nice prints the current scheduling
priority, which it inherited. Otherwise, nice runs the given
command with its scheduling priority adjusted. If no
adjustment is given, the priority of the command is incremented by
10. You must have appropriate privileges to specify a negative
adjustment. The priority can be adjusted by nice over the range
of -20 (the highest priority) to 19 (the lowest).
Synopsis:
nice [ option ]... [ command [ arg ]... ]
Because most shells have a built-in command by the same name, using the unadorned command name in a script or interactively may get you different functionality than that described here.
The program accepts the following option. Also see section Common options.
nohup: Run a command immune to hangups
nohup runs the given command with hangup signals ignored,
so that the command can continue running in the background after you log
out.
Synopsis:
nohup command [ arg ]...
Also, the scheduling priority is increased by 5. If standard output is a tty, it and standard error are redirected so that they are appended to the file `nohup.out'; if that cannot be written to, they are appended to the file `$HOME/nohup.out'. If that cannot be written to, the command is not run.
If nohup creates either `nohup.out' or
`$HOME/nohup.out', it creates it with no "group" or "other"
access permissions. It does not change the permissions if the output
file already existed.
nohup does not automatically put the command it runs in the
background; you must do that explicitly, by ending the command line
with an `&'.
The only options are `--help' and `--version'. See section Common options.
su: Run a command with substitute user and group id
su allows one user to temporarily become another user. It runs a
command (often an interactive shell) with the real and effective user
id, group id, and supplemental groups of a given user.
Synopsis:
su [ option ]... [ user [ arg ]... ]
If no user is given, the default is root, the super-user.
The shell to use is taken from user's passwd entry, or
`/bin/sh' if none is specified there. If user has a
password, su prompts for the password unless run by a user with
effective user id of zero (the super-user).
By default, su does not change the current directory.
It sets the environment variables HOME and SHELL
from the password entry for user, and if user is not
the super-user, sets USER and LOGNAME to user.
By default, the shell is not a login shell.
Any additional args are passed as additional arguments to the shell.
GNU su does not treat `/bin/sh' or any other shells specially
(e.g., by setting argv[0] to `-su', passing -c only
to certain shells, etc.).
su can optionally be compiled to use syslog to report
failed, and optionally successful, su attempts. (If the system
supports syslog.) However, GNU su does not check if the
user is a member of the wheel group; see below.
The program accepts the following options. Also see section Common options.
-c option instead of starting an interactive shell.
-f option to the shell. This probably only makes sense
if the shell run is csh or tcsh, for which the -f
option prevents reading the startup file (`.cshrc'). With
Bourne-like shells, the -f option disables file name pattern
expansion (globbing), which is not likely to be useful.
TERM, HOME, and SHELL
(which are set as described above), and USER and LOGNAME
(which are set, even for the super-user, as described above), and set
PATH to a compiled-in default value. Change to user's home
directory. Prepend `-' to the shell's name, intended to make it
read its login startup file(s).
HOME, USER,
LOGNAME, or SHELL. Run the shell given in the environment
variable SHELL instead of the shell from user's passwd
entry, unless the user running su is not the superuser and
user's shell is restricted. A restricted shell is one that
is not listed in the file `/etc/shells', or in a compiled-in list
if that file does not exist. Parts of what this option does can be
overridden by --login and --shell.
su is not the superuser and user's
shell is restricted (see `-m' just above).
Why GNU su does not support the `wheel' group
(This section is by Richard Stallman.)
Sometimes a few of the users try to hold total power over all the rest. For example, in 1984, a few users at the MIT AI lab decided to seize power by changing the operator password on the Twenex system and keeping it secret from everyone else. (I was able to thwart this coup and give power back to the users by patching the kernel, but I wouldn't know how to do that in Unix.)
However, occasionally the rulers do tell someone. Under the usual
su mechanism, once someone learns the root password who
sympathizes with the ordinary users, he or she can tell the rest. The
"wheel group" feature would make this impossible, and thus cement the
power of the rulers.
I'm on the side of the masses, not that of the rulers. If you are used to supporting the bosses and sysadmins in whatever they do, you might find this idea strange at first.